Early blight is a fungal disease, Alternaria sp., that occurs on tomatoes throughout North America. Early blight can affect seedlings but is generally observed on older plants and is especially severe on plants of poor vigor. Plants infected with the fungus can display collar rust on the stems, infected older leaves, and fruits that crack at the stem. Infection on leaves is the most common symptom.

Symptoms and Diagnosis

The appearance of circular or irregular dark spots on the lower, more mature leaves is one of the first symptoms of infection. Eventually, the spots enlarge into a series of concentric rings surrounded by a yellow area. The entire leaf may be killed and will drop off the plant. Early blight can result in extensive defoliation, exposing fruit to sunscald and reducing yields. This disease typically progresses from the base of the plant, upward.

Life Cycle

Early blight spores survive on old plant debris or in the soil. Spores are spread by wind and rain, but occasionally, flea beetles transmit this disease. Fungal spores enter a host through wounds in the plant cuticle. Spores thrive in moist, warm temperatures (80–90 degrees F) and can persist in partially decomposed garden waste for at least a year.

Integrated Pest Management Strategies

1. Plant resistant varieties. Varieties such as ‘Early Cascade’, ‘Floramerica’, ‘Jetstar’, ‘Manlucie’, ‘Supersonic’, and ‘Surecrop’ have some tolerance to early blight. These varieties will require a less intensive management program than susceptible varieties.

2. Maintain plant vigor. Stressed plants are more susceptible to early blight. Water the plants regularly, but don't fertilize until the plants are well-established and in full blossom. Do not mulch until the soil is warm.

3. Do a thorough cleanup of the garden in the fall. Remove plant debris or till it into the soil. Pull weeds that compete for light, water, and nutrients, especially nightshade, horse nettle, and other weeds in the tomato family.

4. Rotate crops. Practice a 2- or 3-year crop rotation. Avoid planting eggplant or potatoes where tomatoes were last planted.

5. Avoid activity when plants are wet. Confine staking and picking to times when foliage is dry. Disease is more readily spread when plant foliage is wet.